[time-nuts] Broken Ovenaire OSC 85-50

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jul 4 04:05:17 UTC 2009


You can estimate the crystal current by measuring the RF voltage across
the 15pF cap in the oscillator circuit and estimate/measure the varactor
and trimmer capacitors.
Alternatively, temporarily replace all these caps with a 33pF cap
connected from the inductor in series with crystal and ground.
33pf has a reactance of ~500 ohm at 10MHz.
Use a relatively low capacitance probe for this and account for the
probe capacitance in the calaculations.

This should at least allow you to determine if the crystal current is
around  10uA, 100uA or 1mA or more.

Bruce

J. L. Trantham wrote:
> I did not realize that this post would generate this much discussion.  
>
> While I have a BEE and MSEE, I am a practicing Cardiac Electrophysiologist
> and it has been some time since I thought about such issues.  I have learned
> a lot though from the discussions and as medicine as we know it winds down,
> interests in EE will grow.
>
> I agree with Warren.  It should be fixed before modified.  However, I could
> not find anything that was 'broken' with the possible exception of the XTAL.
> As I tried various things, I did notice from time to time some reluctance to
> oscillate suggesting that the oscillator stage was an issue.  With no
> obvious failure and no hope of finding a replacement XTAL, I had to choose
> other options.
>
> The real problem is size.  There is no room for added components, at least
> on the Oscillator Board which is inside a sealed oven.  There is only room
> for changing components.  I would have liked to try adding a resistor and
> capacitor to the emitter circuit of the amplifier transistor on the
> Oscillator Board but there is no room.  I have pictures if anyone is
> interested.  The Oscillator Board is inside a metal can about 0.5x1.5x2.0 cm
> that has a heater coil wrapped around the outside and an Oven Controller
> Board attached to the outside.  This package is surrounded by foam with the
> Output Board on one face of the foam, mounted to one wall of the outside
> metal package.  The bottom of the metal package has 2 feed thru's, plus a
> ground connection.  One is 5 VDC and the other is the 10MHz out.
>
> While with better equipment one might be able to find a problem, it was
> certainly not obvious to me.
>
> Do XTAL's fail often?  If so, by what mechanism?  Slowly fade away?  Sudden
> death?  I am not experienced enough to know this answer but in all my years
> with various HAM gear, I do not recall the failure of a single XTAL.
> However, none of these were in an oven.
>
> Thanks again for all the useful information.
>
> Joe
>
>
>
> None the less, with a simple change of resistor, the gain in the amplifier
> stage increased and, at least so far, problem solved.
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of WarrenS
> Sent: Friday, July 03, 2009 7:49 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>
>
> Here is my  two cents worth
> 20 mv output, sure sounds like something is broken.
> It should be fixed before it is modified.
> The 2.49 volts on the Red input voltage seem LOW, More Vcc maybe. The "Grn"
> labeled wire, FreqCtrl  input should be about 1/2 VCC  for testing. If you
> do 'need' to modify the gain, 
> It would seem better to bypass the 470 ohm resistor with a cap in series
> with the 47 ohms.
>
> ws
> ********************
> ----- Original Message ----- 
> From: "Mike Monett" <xde-l2g3 at myamail.com>
> To: <time-nuts at febo.com>
> Sent: Friday, July 03, 2009 11:50 AM
> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>
>
>   
>>  > An update on the Broken Ovenaire OSC 85-50.
>>
>>  > I prepared  a 'schematic' of the Output Board  and  the Oscillator  
>>     
>>> Board (attached) and I have lots of pictures of the  external unit  
>>> and the insides if anyone is interested.
>>>       
>>  > I resoldered  all connections and replaced all transistors  on the  
>>     
>>> Output Board  and  the  Oscillator  Board  all  to  no  benefit. I  
>>> measured all  the components with an LCR meter and found  the 0.01  
>>> uF bypass  on the 330 ohm resistor in the emitter  circuit  of the  
>>> output transistor  of the Output Board to be low and  with  a high  
>>> ESR. I replaced this with about a 20% increase in output amplitude  
>>> but still  inadequate. I replaced the rest of the 0.01 uF  caps on
>>>       
>>  > the output  board  with   no   additional  benefit.  I transiently
>>  > disconnected the  Red  wires from the  Oven  Controller  board and  
>>     
>>> there was no increase in output or significant increase in voltage  
>>> to the Oscillator Board.
>>>       
>>  > Therefore, it  appeared  that a 'low output  crystal'  (if  such a  
>>     
>>> thing exists)  was the only logical explanation that I  could come  
>>> up with. That seeming to be the case, there appeared to be  only 4  
>>> options. 1. Toss the OCXO (sorry, too much effort so far). 2.
>>>       
>>  > Build an  external   amplifier   (seemingly   too  much additional
>>  > effort). 3. Try to adjust on the bias of the oscillator transistor  
>>     
>>> to achieve  a higher output (seemed too 'iffy'). Or  4.  Lower the  
>>> value of  the  resistor in the emitter circuit  of  the Oscillator  
>>> Board to  get  more gain out of the last stage  in  the Oscillator  
>>> Board.
>>>       
>>  > I replaced  the  470 ohm resistor with a 47 ohm  resistor  and the  
>>     
>>> amplitude increased to about 0.4 V P-P into a 50 ohm load  and was  
>>> sufficient to make it a usable OCXO again.
>>>       
>>  > I reassembled, resealed with Epoxy and all seems well so far.
>>
>>  > If anyone wants pictures or other info, please let me know.
>>
>>  > Thanks for all the suggestions and help.
>>
>>  > Joe
>>
>>  Joe,
>>
>>  Congratulations on getting your system to work!
>>
>>  A couple  of  things. First, trying to measure the  currents  in the  
>> circuit with a ferrite toroid won't do you much good. You don't know  
>> what the  currents  should  be,  and  the  secondary  of  the toroid  
>> transformer requires a termination resistor. The value  changes with  
>> the turns ratio.
>>
>>  Just from looking at the circuit, the RF currents will  be extremely  
>> low. This  requires a large number of turns on the  secondary, which  
>> will probably resonate at or below the 10MHz operating frequency due  
>> to stray  capacitance  from the connection to the  scope.  So  it is  
>> unlikely you will get any useful progress in this direction.
>>
>>  However, from the values on your schematic, the output  tank circuit  
>> resonates at  9.602MHz with a Q of 9.6. So the tank is  already well  
>> below resonance, which attenuates the output voltage.
>>
>>  Any stray capacitance you add to the circuit will bring the resonant  
>> frequency lower, further aggravating the loss in signal.
>>
>>  The output  tank  is tapped with the 75pF and 91pF  in  series. This  
>> further attenuates the signal.
>>
>>  I'd change the circuit to a single capacitor across the tank  with a  
>> small trim capacitor to tune it to resonance.
>>
>>  To get  the signal into 50 ohms for distribution, I'd add  a limiter  
>> if you can tolerate a square wave output, or a good emitter follower  
>> if you  need a sine wave. Take the output from the collector  of the  
>> 2N2369 to get the maximum signal amplitude.
>>
>>  Your original  post  mentions an output amplitude  of  20mV.  If the  
>> normal amplitude is around 2V, this represents a loss of  40dB. This  
>> is a huge loss in signal. The circuit obviously worked at  one time,  
>> so there may well be some other hidden problem.
>>
>>  It is  possible the crystal is damaged, but this  seems  unlikely. A  
>> crystal oscillator probably won't even start if the signal  level is  
>> down 40dB.
>>
>>  You can  check  the oscillator and crystal in  SPICE.  Normally, the  
>> high Q  of  the crystal will make the analysis very  slow.  It could
>>  take many  hours  for   the   simulation  to  begin  oscillating and
>>  stabilize at the final amplitude. The transient analysis  requires a  
>> very fine  time step for accuracy, and you could run  out  of memory  
>> before the simulation was complete.
>>
>>  I have developed a much faster way of analyzing a crystal oscillator  
>> in SPICE.  Instead  of requiring tens or  hundreds  of  thousands of  
>> simulated cycles,  this method gives accurate results in only  a few  
>> dozen cycles.  For more information, please see  "SPICE  Analysis of  
>> Crystal Oscillators"
>>
>>  http://pstca.com/spice/xtal/clapp.htm
>>
>>  You can  estimate the value of the crystal ESR by finding  the  Q of  
>> your crystal and working backwards.
>>
>>  I'm attaching a gif of your schematic for reference. This is rotated  
>> 90 degrees and enhanced in LView Pro to improve the contrast.
>>
>>  Please let me know if you have any questions.
>>
>>  Thanks,
>>
>>  Mike
>>
>>     
>
>
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